Certain chromium compounds are environmentally and occupationally important human respiratory toxins and carcinogens. The forms of chromium most frequently associated with human exposure, toxicity and carcinogenesis are particulate salts of low solubility (lead chromate), which deposit and persist in the respiratory tract after inhalation. Lead chromate particles are genotoxic to cultured fibroblasts, but although the particles can be phagocytized, the genotoxicity is dependent on extracellular dissolution of particles at the cell surface microenvironment which is markedly accelerated by particle-cell contact. The particle-derived dissolved hexavalent chromate oxyanion enters the cell where it undergoes a uniquely complex scheme of reductive metabolism, producing a cascade of reactive intermediates each of which appears to produce a specific form of DNA damage. The spectrum of DNA damage includes adduct formation, strand breaks, chromosomal aberrations, DNA-protein crosslinks and DNA-DNA crosslinks which constitute a base-specific polymerase arresting lesion both in vitro and in vivo. Intracellular reduction of chromium also produces reactive oxygen species. The combination of direct DNA damage and indirect oxidative stress alters DNA replication and transcription, interferes with the cell cycle and provokes cell commitment to apoptotic cell death, terminal differentiation, or neoplastic transformation. The DNA damage and cell fate endpoints can be markedly altered by antioxidants suggesting the partial involvement of oxidative damage. The overall objective of this continuation application is to investigate the interaction of particulate chromium compounds with human respiratory epithelial cells which are the main targets of chromium toxicity and carcinogenesis. We will examine the mechanisms of cellular toxicity and their role in chromium-induced cell fate, focusing on DNA replication and oxidative stress. We will test the following hypotheses: 1) that particulate chromium compounds will cause genotoxicity and apoptotic cell death, in human respiratory epithelial and stromal cells, and to determine whether the genotoxicity is dependent on particle-cell contact, phagocytosis, and/or cell-enhanced dissolution of particles in the cell microenvironment, 2) that the toxicity and genotoxicity of particulate chromium compounds in human respiratory epithelial cells will be modulated by the presence of co-cultured fibroblasts and/or inflammatory cells, 3) that oxidative stress contributes tot he toxicity, genotoxicity and apoptotic cell death caused by particulate chromium compounds in human respiratory epithelial and stromal cells, and 4) that specific reductive chromium metabolites and types of chromium-induced DNA damage differentially affect the process of DNA replication. This research will have practical value in evaluating the relative risk to humans in contact with particulate chromates and will also help elucidate cellular and molecular mechanisms of toxicity and carcinogenesis.